Tension limit modification control



A ril 28, 1970 H. N. cox

I TENSION LIMIT MODIFICATION CONTROL Filed March 7, 1968 INVENTOR HOWARD N. COX

United States Patent O 3,508,425 TENSION LIMIT MODIFICATION CONTROL Howard N. Cox, Scotia, N.Y., assignor to General Electric Company, a corporation of New York Filed Mar. 7, 1968, Ser. No. 711,290 Int. Cl. B21b 37/12 US. Cl. 72-9 4 Claims ABSTRACT OF THE DISCLOSURE Apparatus for adjusting the roll force of a metals rolling strip mill including a digital computer having an output signal representative of predetermined values of strip tension derived from computer inputs of strip speed, strip width and strip thickness (disregarding the nature of the strip material), the output signal being fed to a pair of digital/analog converters calibrated respectively according to preselected high and low limits of strip tension, each of the analog outputs being compared in an amplifier with actual (measured) strip tension and each of the amplifiers being arranged via servomechanisms to change the roll force oppositely according to the deviation of actual strip tension above and below the respective high and low limits.

BACKGROUND OF THE INVENTION In metals rolling mills such as steel mills the efiiciency of rolling varies with the speed of the rolls working the metal. That is, a given reduction in thickness can be achieved, for example, with less roll force and tension at high speeds than at low speeds. This phenomenon is attributed to the increase in friction between the metal strip and the rolls as the speed decreases.

If strip thickness is maintained constant as speed decreases, tension, or roll force, or both, must increase. If the roll force is not changed and the strip thickness is held constant as speed decreases by adjusting speed ratios between adjacent pairs of work rolls, the tension may increase approximately 400% when rolling very thin strip. Consequently, the strip might be torn or pulled apart.

Steel mill operators normally have tension limit controls which increase the roll force of the pairs of rolls downstream from the strip section when the tension exceeds the predetermined upper limit set by the operator. Conversely roll force is decreased when the tension becomes less than the predetermined lower tension limit set by the operator. Normally the operator sets the upper and lower tension limits at values appropriate for high speed conditions of the rolls. When strip speed decreases appreciably operators usually discontinue automatic tension limit control rather than attempt to arbitrarily shift the high and low tension limits.

As the speed decreases the operators manually increase roll force and accept somewhat higher tension values depending upon their judgment and experience as to how much each should be allowed to increase.

SUMMARY OF THE INVENTION The invention provides a means to overcome the excessive roll force resulting from arbitrary substitution of roll force for tension in the use of the automatic tension limit control and avoids the presently used alternative of manual manipulation of substituting roll force for tension through providing an automatic tension limit modification control having a computer which has three input signals applied thereto, representing the width, thickness, and the speed of the strip of material being rolled. The computer provides two outputs derived from three input signals, representing allowable tension limits during predetermined ranges of speed. Of these three signals the width and the predetermined thickness of the strip remain relatively conice stant for each run and are therefore used in the calculation of the total tension prior to the run. The speed, however, provides an input signal to the computer which varies from standstill to top speed. Based on this signal and the two other predetermined signals, the computer produces two digital output signals which adjust a pair of voltage divider bridges so that each produces a voltage representing a maximum and a minimum allowable tension limit respectively which is applied to a pair of amplifiers which also receive a signal from a tensiometer representing the actual total tension developed in the strip of metal being rolled. The resultant signals from these amplifiers are applied to a pair of relays which in turn operate the screwdown controls of the mill and cause a change in roll force when required to offset a change in tension due to a change in speed. The computer is programmed to provide a stepped program for substituting roll force for tension so that both the roll force and the tension are held within allowable limits.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 presents a block diagram of the tension limit modification control showing the relationship between the components.

FIGURE 2 shows a typical resistance bridge for providing stepped digital to analog conversion of signals.

DETAILED DESCRIPTION A signal 11 representing the strip speed derived from a tachometer (not shown) is applied to a first input of a digital computer 15. A second signal 13 representing the strip thickness derived from a thickness gage (not shown) is applied to a second input of computer 15. A third signal 14, representing strip width, is applied to a third input of computer 15. Based on these three input signals the computer produces two signals and applies them to a high tension limit reference circuit 17 and a low tension limit reference circuit 19. The construction of these tension limit reference circuits is described lat-er. The output of the tension limit reference circuits is in the form of an analog voltage signal having a negative polarity and values which are not equal thereby providing a dead band between them. For example, the output of the high tension limit reference circuit 17 may be 7 volts, and the output of the low tension limit reference circuit 19 may be 5 volts, thereby providing a dead band of 2 volts of negative polarity. These two voltages are applied to a first input terminal of two amplifiers 21 and 23, respectively, each of which has applied to a second input terminal a positive potential signal 25 which is proportional to the total tension developed in the strip.

The output from the amplifier 21 is applied to the high tension limit screw adjustment relay coil 29 while the output of the amplifier 23 is applied to the low tension limit screw adjustment relay 27. When the coil 29 of the high tension screw adjustment relay is energized a normally open contact 31 closes and causes current to be applied to the screwdown motor (not shown) Whereby the screws are run down and roll force in the mill is increased. When coil 27 of the low tension screw adjustment relay 27 is energized, a normally open contact 33 closes and current is applied to the screwdown motor which causes the screws to be run up and the roll force to be decreased.

OPERATION The strip speed signal 11 in addition to the predetermined strip thickness and the strip width signals applied to the computer causes an adjustment to be made by the computer in the output of the high and low tension limit reference circuits 17 and 19, respectively. The circuitry involved in these tension limit reference circuits is shown in FIG. 2 where a series of resistors 50-57 are connected in series to form a resistor voltage division bridge between positive and negative power supply busses 59 and 61, respectively. Connected in parallel to this resistor bridge is a series of normally open contacts 63-66 each of which is connected to the junctions between adjacent resistors. For example, normally open contact 63 is at one end connected to the negative bus 61 and at the other end to normally open contact 64 and to the junction between resistors 56 and 57, thereby forming a parallel path to resistor 57. In the same manner normally open contact 64 is connected in parallel to resistor 56 and normally open contact 6-5 connects in parallel to resistor 55. Resistor 54 is shunted by normally open contact 66.

A series of normally closed contacts 71-74 is connected in parallel with resistors 50-53 so that normally closed contact 71 is in parallel with resistor 53, normally closed contact 72 is in parallel with resistor 52, normally closed contact 73 is connected in parallel with resistor 51 and normally closed contact 74 connects in parallel with resistor 50 and to positive bus 59. The value of resistor 54 is the same as the value of resistor 53, similarly the value of resistor 55 is the same as that of resistor 52, and resistor 56 has the same resistance as resistor 51. Resistor 57 is of the same value as resistor 50. The total resistance of the circuit between the positive bus 59 and the negative bus 61 is equal to the sum of the resistances 57-54, since the normally closed contacts 71-74 short out the resistors 50-53. In the circuit normally open contact 66 operated simultaneously with normally closed contact 71, so that when 71 opens normally open contact 66 closes; hence, resistor 53 is placed in the circuit while resistor 54 is removed by being shorted out, each time the contacts 66 and 71 change state. Since both these resistors have the same value, the total resistance between the positive bus 59 and the negative bus 61 remains constant. The voltage level of a tap -81 connected between resistors 53 and 54, however, changes since the resistance between tap 81 and positive bus 59 has been increased and the resistance between tap 81 and negative bus 61 has been decreased. In this example the ,voltage has moved toward the negative bus. Similarly when normally open contact 65 closes, normally closed contact 72 opens and allows the current to pass through resistor 52 While resistor 55 is removed from the circuit. When normally open contact 64 closes, resistor 56 is removed from the circuit and normally closed contact 73 opens and places resistor 51 into the circuit. Normally closed contact 74 opens and allows current to pass through resistor 50 while normally open contact 63 closes and removes resistor 57 from the current path. This circuit, therefore, is a voltage divider circuit and allows a convenient voltage adjustment in steps from a digital output. Each set of normally open and normally closed contacts are part of a relay which is energized by the computer in FIGURE 1 indicated by numeral 15. The voltage thus produced is proportional to the total allowable tension in the strip calculated based on the three input signals to the computer.

For example, assume that a condition exists which causes the computer to apply signals to the high and low tension limit reference circuits 17 and 19, respectively, whereby these circuits produce voltage levels of 7 and volts respectively, and assume that the total tension in the strip is represented by a positive 6 volt signal, then the total voltage applied to the high tension limit reference amplifier 21 is equal to 7 plus 6 resulting 1 volt, and the total voltage applied to the low tension limit reference amplifier 23 is equal to 5+6 resulting in +1 volt. If the total tension increases the positive voltage signal will increase to a point where the amplifier input and its corresponding output voltage Will change polarity and cause the relay coil 29 to become energized. In the above example this will occur when the voltage signal 25 increases above a positive 7 volts resulting in the Output of amplifier 21 causing coil 29 to be energized, thereby closing the normally open contact 31 which energizes the screwdown control circuit to run the screws down so that the tension in the strip is decreased.

Similarly when the tension decreases to a point where the total input voltage to amplifier 23 becomes negative, relay coil 27 will become energized and cause normally open contact 33 to close causing the screws to be run up so that roll force is decreased. In the above example, this will occur when the voltage signal decreases due to a reduction in total tension in the strip being rolled to less than positive 5 volts thereby causing the total input of the amplifier 23 to become negative in polarity and energizing relay 27. As a consequence normally open contact 33 closes, the screwdown control circuit is energized and causes the screws to be run up to increases the tension in the strip being rolled.

It is Well known that with a reduction in speed and all other factors remaining constant in a multi-stand rolling mill, the tension in the strip will increase correspondingly, due to an increase in the coeflicient of friction of the roll bite. While an increase in tension is not desirable it is a phenomenon which presently has to be accepted.

While the invention has been explained and described with the aid of particular embodiments thereof, it will be understood that the invention is not limited thereby and that many modifications retaining and utilizing the spirit thereof without departing essentially therefrom will occur to those skilled in the art in applying the invention to specific operating environments and conditions. It is therefore contemplated by the appended claims to cover all such modifications as fall within the scope and spirit of the invention.

What is claimed is:

1. In a metals rolling mill having pairs of rolls forcibly bearing upon a strip passing therethrough, a digital computer, an input to said computer representative of strip speed, inputs respectively to said computer representative of strip width and strip thickness for producing computer output signals representative of predetermined values of strip tension in respect to difierent strip speeds, a pair of circuits arranged to receive said output signals, means biasing each said circuit input representative of assigned high and low values of strip tension, means for measuring the value of the actual strip tension, means for comparing the output of said circuits with said measured value, and

means controlled oppositely by each said comparing means respectively for changing the roll force upon said strip according to said comparison in response to changes in strip speed.

2. The invention as claimed in claim 1 wherein each of said pair of circuits comprises a digital to analog converter, each arranged to receive said output signals, means for biasing said converter input representative of a diiferent preselected value of strip tension, means for measuring the value of the actual strip tension, an amplifier for each said converter output including means for comparing the outputs of said converters with said measured value, and means controlled oppositely by each of said amplifiers respectively for changing the roll force upon said strip according to said comparison in response to changes in strip speed.

3. The invention as claimed in claim 1 wherein said comparison means comprises a pair of amplifiers each having an input terminal connected to one of said pair of circuits for receiving a first voltage signal and a second input terminal connected to said tension measuring means for receiving a second voltage signal.

4. The invention as claimed in claim 1 wherein said oppositely controlled means comprises a first and second relay connected to said comparing means for receiving one of the output signals and thereby operate to adjust 5 6 the roll force and tension limit in predetermined incre- 3,208,251 9/1965 Hulls et a1. 72-11 mental steps. 3,440,846 4/ 1969 Scott 72--205 X References Cited 3,049,036 8/1962 Wallace et a1. 72-205 5 U3 CL X R 3,169,421 2/1965 Bloodworth 72-234 72 205 3,194,036 7/1965 Canfor et al. 7211 

